1. Field of the Invention
This invention relates to single polarization optical fibers, and particularly to low loss single polarization optical fibers having excellent radiation resistance as well as other improved properties, which fibers may easily be fabricated.
2. Description of the Prior Art
For single polarization optical fibers, various constructions have already been proposed, and the most widely used type has the construction of FIG. 1 which shows an optical fiber of a four-layered construction comprising a core 11, a cladding 12, inner jacket 13 and an outer jacket 14.
The core 11 is made of SiO.sub.2 glass containing a dopant such as GeO.sub.2, P.sub.2 O.sub.5 or the like, the cladding 12 is made of high-purity SiO.sub.2, the inner jacket 13 is made of SiO.sub.2 glass containing P.sub.2 O.sub.5 and B.sub.2 O.sub.3, and the outer jacket 13 is composed of an industrial silica glass tube. In this case, the reason for adding the dopant such as GeO.sub.2, P.sub.2 O.sub.5 or the like to the core 11 is in that it results in an increase of refractive index of the core so that the difference between the refractive index of the core and the refractive index of the cladding is relatively large, whereby the critical angle (maximum angle of incidence) is increased.
Such single polarization optical fibers have very excellent characteristics, but there is a defect in radiation resistance to some extent.
Namely, a dopant such as Ge, P or the like contained in the core is remarkably affected by radiation so that an increase in transmission loss may occur when the single polarization optical fibers are exposed to radiation, for example, in the vicinity of a nuclear reactor.
As a method for fabricating single polarization optical fibers, a soot deposition method similar to the VAD method or inner CVD (Chemical Vapor Deposition) method (or MCVD method, i.e., Modified Chemical Vapor Deposition method) is well known. In optical fibers fabricated in accordance with the inner CVD method, a dopant such as GeO.sub.2, P.sub.2 O.sub.5 or the like contained in the core may vaporize during heating to cause a decrease in the refractive index at the central portion of the core, so that such single polarization optical fiber provides a refractive index distribution as shown in FIG. 2. Otherwise, impurities contained in its outer jacket may be fused during heating to become united in the core so that satisfactory characteristics would not necessarily be obtained.
According to the results of our studies and experiments, it has been discovered that the core is desirably made from either high-purity SiO.sub.2 or high-purity SiO.sub.2 containing trace amounts of B.sub.2 O.sub.3 in order to improve radiation resistance.
However, when such materials as mentioned above are utilized for a core, not only does the difference in refractive index between the core and its outer jacket become small or nonexistent, but also the difference between the refractive index of the cladding and the refractive index of core is reduced and the difference between the refractive index of the inner jacket and the refractive index of the core is reduced. As a result, the transmission band may become narrower. In order to improve this deficiency, the total thickness of the cladding and the inner jacket may be increased. However, if the total thickness of the cladding and the inner jacket is increased, the outer diameter of the resulting optical fiber may become larger than that of a conventional optical fiber that the utility thereof decreases.
In the case where the core of an optical fiber is constituted of high-purity SiO.sub.2 or high-purity SiO.sub.2 containing trace amounts of B.sub.2 O.sub.3, it is desirable to make the critical angle (maximum angle of incidence) large. For this reason, it is necessary for the cladding to select a material having a lower refractive index than that of SiO.sub.2 for constituting the optical fiber, so that such requirement is a significant problem in actuality.
Namely, it is convenient that the outermost layer of an optical fiber is made of high-purity SiO.sub.2 in view of the fabrication thereof. Hence, if such fact is applied to a single polarization optical fiber, a refractive index distribution as indicated in FIG. 9 is obtained.
As a matter of course, it is not required that the outermost layer of high-purity silica is as pure as the core, but since the outermost layer does not contain significant amounts of dopants, the refractive index thereof becomes substantially equal to that of the core. In such an optical fiber, some of the energy of a wave traveling in the core may be transferred to the outermost layer. Thus, a spacing .delta.T between the core and the outer jacket (support tube) must be widened in order to reduce attenuation due to leakage. Specifically speaking, when the normalized frequency (=n.sub.o k.sub.o T2.DELTA., wherein n.sub.o : refractive index of core, .lambda.: applied wavelength, T: radius of core, and .DELTA.: difference in refractive indices of core and inner jacket) is 2.2, it is required to satisfy such condition that .delta.&gt;6 in the minor axis of the inner jacket. Owing to the above reason, for the sake of fabrication of a low loss single polarization optical fiber having a high ellipticity, it is necesary to increase the difference in the refractive indices .DELTA. of the core and the inner jacket, while decreasing the diameter of the core. On the other hand, however, the transmission loss as well as the single operation wavelength band are remarkably influenced by variation in the core diameter in such construction as described above so that it is difficult to fabricate single polarization optical fibers having a stable single mode operation wavelength band in the longitudinal direction thereof. A band in which an optical fiber can be operated in single mode by decreasing leakage may be expressed by approximately 2.0&lt; v&lt;2.3 in accordance with normalized frequency v, if the variation in the diameter of the optical fiber along the longitudinal direction thereof is taken into consideration. Accordingly, there is such a problem that the applicable wavelength band of the type of optical fiber as described above is limited to a narrow range. Furthermore, in the case where, for example, the difference in the refractive indices .DELTA. of the core and the inner jacket is 0.4% and the wavelength is 0.85 .mu.m, the core diameter must be 4.98 .mu.m even at the maximum, and where the wavelength is 0.63 .mu.m, the core diameter is limited to 3.71 .mu.m at the maximum.
In such construction as described above of the optical fiber, there have been such problems that it is difficult to fabricate low loss single polarization optical fibers being homogeneous along the longitudinal direction thereof, because loss is significantly influenced by variation in the core diameter of the optical fiber. Moreover, it is also difficult to connect such optical fibers to one another, or the optical fibers with a light source, because the core diameter of such optical fibers is too small.